Fast acquisition of a pilot signal in a wireless communication device

ABSTRACT

In a remote unit of a wireless communication system, the speed of acquisition, or reacquisition, of a pilot signal by a search engine is increased. PN space is divided into segments and a coarse search of each segment is performed using a set of “fast” search parameters. A set of peak signal strengths along with their corresponding PN offsets, is saved for each segment of PN space. Following the search of all segments of PN space, the peak signal strengths which were saved during the coarse acquisition are evaluated. Coarse acquisition search results are used by the remote unit to determine fine search window parameters for subsequent searches performed by the remote unit search engine. The fine search parameters concentrate searching efforts on portions of PN space most likely to contain a viable base station pilot signal.

RELATED APPLICATIONS

The following U.S. Patent Applications filed concurrently herewith arerelated to this application and are hereby incorporated by reference intheir entirety: REACQUISITION AND HANDOFF IN A SLOTTED MODECOMMUNICATION SYSTEM, U.S. patent application Ser. No. 09/540,801;EFFICIENT SEARCHING BY A REMOTE UNIT IN A SLOTTED MODE COMMUNICATIONSYSTEM, U.S. patent application Ser. No. 09/540,800; PRIORITIZATION OFSEARCHING BY A REMOTE UNIT IN A SLOTTED MODE COMMUNICATION SYSTEM, U.S.patent application Ser. No. 09/540,802.

FIELD OF THE INVENTION

The invention relates to wireless communications systems. In particular,the invention relates to acquisition of a pilot signal in a wirelesscommunication system.

BACKGROUND OF THE INVENTION

A wireless communication system may comprise multiple remote units andmultiple base stations. FIG. 1 exemplifies an embodiment of aterrestrial wireless communication system with three remote units 10A,10B and 10C and two base stations 12. In FIG. 1, the three remote unitsare shown as a mobile telephone unit installed in a car 10A, a portablecomputer remote 10B, and a fixed location unit 10C such as might befound in a wireless local loop or meter reading system. Remote units maybe any type of communication unit such as, for example, hand-heldpersonal communication system units, portable data units such as apersonal data assistant, or fixed location data units such as meterreading equipment. FIG. 1 shows a forward link 14 from the base station12 to the remote units 10 and a reverse link 16 from the remote units 10to the base stations 12.

Communication between remote units and base stations, over the wirelesschannel, can be accomplished using one of a variety of multiple accesstechniques which facilitate a large number of users in a limitedfrequency spectrum. These multiple access techniques include timedivision multiple access (TDMA), frequency division multiple access(FDMA), and code division multiple access (CDMA). An industry standardfor CDMA is set forth in the TIA/EIA Interim Standard entitled “MobileStation—Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny(collectively referred to here as IS-95), the contents of which areincorporated by reference herein in their entirety. Additionalinformation concerning a CDMA communication system is disclosed in U.S.Pat. No. 4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS, (the '307patent) assigned to the assignee of the present invention andincorporated in its entirety herein by reference.

In the '307 patent, a multiple access technique is disclosed where alarge number of mobile telephone system users, each having atransceiver, communicate through base stations using CDMA spreadspectrum communication signals. The CDMA modulation techniques disclosedin the '307 patent offer many advantages over other modulationtechniques used in wireless communication systems such as TDMA and FDMA.For example, CDMA permits the frequency spectrum to be reused multipletimes, thereby permitting an increase in system user capacity.Additionally, use of CDMA techniques permits the special problems of theterrestrial channel to be overcome by mitigation of the adverse effectsof multipath, e.g. fading, while also exploiting the advantages thereof.

In a wireless communication system, a signal may travel several distinctpropagation paths as it propagates between base stations and remoteunits. The multipath signal generated by the characteristics of thewireless channel presents a challenge to the communication system. Onecharacteristic of a multipath channel is the time spread introduced in asignal that is transmitted through the channel. For example, if an idealimpulse is transmitted over a multipath channel, the received signalappears as a stream of pulses. Another characteristic of the multipathchannel is that each path through the channel may cause a differentattenuation factor. For example, if an ideal impulse is transmitted overa multipath channel, each pulse of the received stream of pulsesgenerally has a different signal strength than other received pulses.Yet another characteristic of the multipath channel is that each paththrough the channel may cause a different phase on the signal. Forexample, if an ideal impulse is transmitted over a multipath channel,each pulse of the received stream of pulses generally has a differentphase than other received pulses.

In the wireless channel, the multipath is created by reflection of thesignal from obstacles in the environment such as, for example,buildings, trees, cars, and people. Accordingly, the wireless channel isgenerally a time varying multipath channel due to the relative motion ofthe structures that create the multipath. For example, if an idealimpulse is transmitted over the time varying multipath channel, thereceived stream of pulses changes in time delay, attenuation, and phaseas a function of the time that the ideal impulse is transmitted.

The multipath characteristics of a channel can affect the signalreceived by the remote unit and result in, among other things, fading ofthe signal. Fading is the result of the phasing characteristics of themultipath channel. A fade occurs when multipath vectors adddestructively, yielding a received signal that is smaller in amplitudethan either individual vector. For example if a sine wave is transmittedthrough a multipath channel having two paths where the first path has anattenuation factor of X dB, a time delay of δ with a phase shift of Θradians, and the second path has an attenuation factor of X dB, a timedelay of δ with a phase shift of Θ+π radians, no signal is received atthe output of the channel because the two signals, being equal amplitudeand opposite phase, cancel each other. Thus, fading may have a severenegative effect on the performance of a wireless communication system.

A CDMA communication system is optimized for operation in a multipathenvironment. For example, the forward link and reverse link signals aremodulated with a high frequency pseudonoise (PN) sequence. The PNmodulation allows the many different multipath instances of the samesignal to be separately received through the use of a “rake” receiverdesign. In a rake receiver, each element within a set of demodulationelements can be assigned to an individual multipath instance of asignal. The demodulated outputs of the demodulation elements are thencombined to generate a combined signal. Thus, all of the multipathsignal instances must fade together before the combined signalexperiences a deep fade.

In a communication system based on the industry standard for CDMA,IS-95, each of the multiple base stations transmits a pilot signalhaving a common PN sequence. Each base station transmits a pilot signaloffset in time from neighboring base stations so that the signals can bedistinguished from one another at the remote unit. At any given time,the remote unit may receive a variety of pilot signals from multiplebase stations. Using a copy of the PN sequence produced by a local PNgenerator, the entire PN space can be searched by the remote unit. Usingthe search results, the controller distinguishes pilot signals frommultiple base stations based on the time offset.

In the remote unit, a controller is used to assign demodulation elementsto the available multipath signal instances. A search engine is used toprovide data to the controller concerning the multipath components ofthe received signal. The search engine measures the arrival time andamplitude of the multipath components of a pilot signal transmitted bythe base stations. The effect of the multipath environment on the pilotsignal and the data signal transmitted by a common base station is verysimilar because the signals travel through the same channel at the sametime. Therefore, determining the multipath environment's effect on thepilot signal allows the controller to assign demodulation elements tothe data channel multipath signal instances.

The search engine determines the multipath components of the pilotsignals of base stations in the proximity of the remote unit bysearching through a sequence of potential PN offsets and measuring theenergy of the pilot signal received at each of the potential PN offsets.The controller evaluates the energy associated with a potential offset,and, if it exceeds a threshold, assigns a signal demodulation element tothat offset. A method and apparatus of demodulation element assignmentbased on searcher energy levels is disclosed in U.S. Pat. No. 5,490,165entitled DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OFRECEIVING MULTIPLE SIGNALS (the '165 patent), assigned to the assigneeof the present invention.

FIG. 2 shows an exemplifying set of multipath signal instances of asingle pilot signal from a base station arriving at a remote unit. Thevertical axis represents the power received in decibels (dB). Thehorizontal axis represents the delay in the arrival time of a signalinstance due to multipath delays. The axis (not shown) going into thepage represents a segment of time. Each signal spike in the common planeof the page has arrived at the remote unit at a common time but has beentransmitted by the base station at a different time. Each signal spike22-27 has traveled a different path and therefore exhibits a differenttime delay, a different amplitude, and a different phase response. Thesix different signal spikes represented by spikes 22-27 arerepresentative of a severe multipath environment. A typical urbanenvironment produces fewer usable paths. The noise floor of the systemis represented by the peaks and dips having lower energy levels. Thetask of the search engine is to identify the delay, as measured by thehorizontal axis, and amplitude, as measured by the vertical axis, ofsignal spikes 22-27 for potential demodulation element assignment.

Note, as shown in FIG. 2, each of the multipath peaks varies inamplitude as a function time as shown by the uneven ridge of eachmultipath peak. In the limited time shown, there are no major changes inthe multipath peaks. Over a more extended time range, multipath peaksdisappear and new paths are created as time progresses. Multipath peaksare likely to merge together or blur into a wide peak over time.

Typically, the operation of the search engine is overseen by acontroller. The controller commands the search engine to step through aset of offsets, called a search window, that is likely to contain one ormore multipath signal peaks suitable for assignment to a demodulationelement. For each offset, the search engine reports the energy it foundoffset back to the controller. Demodulation elements may then beassigned by the controller to the paths identified by the search engine(i.e. the timing reference of their PN generators is aligned with thetiming of the identified path). Once a demodulation element has lockedonto the signal, it then tracks that path on its own without controllersupervision, until the path fades away or until the demodulation elementis assigned to another path by the controller.

As noted above, each base station in a given geographical area isassigned a sequence offset of a common PN pilot sequence. For example,according to IS-95, a PN sequence having 2¹⁵ chips and repeating every26.66 milliseconds (ms) is transmitted by each base station in thesystem at one of 512 PN sequence offsets as a pilot signal. According toIS-95 operation, the base stations continually transmit the pilot signalwhich can be used by the remote unit to identify the base station aswell as other functions, such as for example, determining the multipathenvironment the remote unit is operating in and synchronization ofremote unit timing to the base station timing.

During initial power on, or any other situation when the remote unit haslost a pilot signal such as when performing a hard hand-off to adifferent operating frequency, the remote unit evaluates all possible PNoffsets of the pilot PN sequence. Typically, a search engine measuresthe pilot signal strength at all possible PN offsets, proceeding at ameasurement rate that produces an accurate measure of the pilot signalpresent at the corresponding offset. Proceeding in this manner, thesearch engine determines the PN offset of base stations which aregeographically near the remote unit. Searching each PN offset in thismanner can take anywhere from hundreds of milliseconds to a few secondsdepending on the channel conditions during acquisition. This amount oftime for the remote unit to reacquire a pilot signal is detrimental tothe remote unit operation, and may be annoying to the user of the remoteunit.

FIG. 3 shows an extended portion of PN space on the horizontal axis. Thegroups of peaks 30, 32 and 34 represent transmissions from threedifferent base stations. As shown, the signal from each base stationsignal experiences a different multipath environment. Also, each basestation has a different PN offset from the PN reference 36. Thus, thecontroller may select a set of PN offsets corresponding to searchwindows for any of the identified base stations. This allows the remoteunit to simultaneously demodulate signals from multiple base stations byassigning demodulation elements appropriately.

One purpose of the search engine, as described above, is to aid inmaintaining the wireless link between the remote unit and a basestation. Because the wireless channel is constantly changing, multi-pathsearching must be performed on a virtually continuous basis, otherwisethe wireless link may be lost due to, for example, fading. If thewireless link is lost, communications between the remote unit and thebase station are terminated and the remote unit performs a search of theentire PN space to reacquire an appropriate base station. Searching theentire PN space consumes an excessive amount of time.

Therefore, there is a need in the art for a method of and apparatus tominimize the amount of time required for a remote unit, in a wirelesscommunication system, to acquire or reacquire a pilot signal.

SUMMARY OF THE INVENTION

The invention increases the speed of acquisition, or reacquisition, of apilot signal in a wireless communication system. The wirelesscommunication system comprises a plurality of base stations, eachconfigured to transmit a PN encoded pilot signal at a different timeoffset than the other base stations. In addition there is at least oneremote unit that is configured to perform a search of the PN space.

In one embodiment, PN space is divided into segments. During a fastacquisition of a pilot signal, each of the segments of PN space issearched with “coarse,” fast search engine parameters adjusted toincrease the search rate. A set of peak signal strengths, along withtheir corresponding PN offsets, are saved for each segment of PN space.Following the search of segments of PN space, the peak signal strengthswhich were saved during the coarse acquisition are evaluated. Theresults of the coarse search are used to select “fine” search parametersfor use in a second search of selected portions of the PN space. Thefine search parameters concentrate searching efforts on portions of PNspace most likely to contain a viable base station pilot signal, suchas, for example, portions of the PN space with the highest measuredsignal strength during the coarse search.

Coarse search parameters may comprise a number of non-coherent passesthat is less than the number of passes in the set of fine searchparameters. In addition, the coarse search parameters may comprise anintegration interval that is less than that in the set of fine searchparameters.

Use of coarse search parameters by the remote unit can reduce theduration of the coarse search. In contrast use of fine search parametersproduces more accurate and reliable results. However, fine searchparameters result in a longer search period for a given portion of PNspace that coarse search parameters. Therefore, using coarse searchparameters to search the PN space, and then concentrating on selectedportions of the PN space using fine search parameters, results in ashorter overall search duration while still achieving satisfactorysearch results.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout, and wherein:

FIG. 1 is a representative diagram showing a typical modern wirelesscommunication system.

FIG. 2 is a graph showing an exemplifying set of multiple signalinstances of a pilot signal from a single base station arriving at aremote unit.

FIG. 3 is a graph showing an exemplifying set of multiple signalinstances of pilot signals from multiple base stations arriving at aremote unit.

FIG. 4 is a block diagram of one embodiment of a remote unit.

FIG. 5 is a chart representing PN space divided into segments.

FIG. 6 is a graph showing an exemplifying set of multiple signalinstances of pilot signals from multiple base stations relative to PNspace segments arriving at a remote unit.

FIG. 7 is a graph showing an expanded view of an exemplifying set ofpilot signals from a base stations relative to PN space segmentsarriving at a remote unit.

FIG. 8 is a flowchart showing operation of the fast acquisition of apilot signal.

DETAILED DESCRIPTION OF THE INVENTION

In many wireless communication systems such as, for example, IS-95 CDMAsystems, a pilot signal is used to coordinate communication between abase station and a remote unit. The pilot signal is used, for example,to determine the multipath environment the remote unit is operating in,to synchronize timing between the base station and remote unit, and tomonitor the signal strength from multiple base stations. According toIS-95, each base station transmits a pilot signal encoded with a commonPN sequence and a unique offset. The common PN sequence is 2¹⁵ chips(32,768 chips) in length. The set of all possible PN offsets make up thePN space.

FIG. 4 shows a block diagram of one embodiment of a remote unit 39 foruse in the invention. In this embodiment an RF receiver 40 has an inputport 41 configured to receive RF transmissions. The RF receiver 40 hasan output port 42 coupled to an input port 43 of a search engine 44 andan input port 45 of a demodulator 46, respectively. The search engine 44has an output port 47 coupled to an input port 48 of a memory 49 forstoring search results. A controller 50 which has control outputs 51 and52 coupled to control inputs 53 and 54 of the search engine 44 and thedemodulator 46, respectively. In addition, controller 50 input port 55is coupled to the memory 49 output port 56 so as to provide thecontroller 50 access to the search results stored in the memory 49 bythe search engine 44.

In the embodiment of FIG. 4, the RF receiver 40 receives a transmissionon the input port 41 from a base station (not shown). The output of theRF receiver 40 is passed via the output port 42 to the search engine 44and the demodulator 46, respectively. The operations of the searchengine 44 and demodulator 46 are controlled by the controller 50. Inparticular, controller 50 passes search parameters via output port 51 toinput port 53 of the search engine 44. In addition, controller 50 passescontrol parameters via output port 52 to demodulator 46 input port 54for assignment of demodulator elements within the demodulator 46 in themanner disclosed in the '165 patent. The controller 50 may be amicroprocessor, application specific integrated circuit (ASIC), discretelogic, an analog controller or other control circuitry. The searchengine 44 searches PN space, according to the search parameters, andplaces the search results in memory 49. The controller 50 evaluates thesearch results stored in memory 49 for assignment of elements of thedemodulator 46 and for determination of future search parameters.

When the remote unit is initially powered on, the controller 50 createsa set of fast search parameters. The search parameters comprise a searchwindow, an integration interval and a number of non-coherent passes, asdescribed in U.S. patent application 09/346,369, entitled DYNAMICALLOCATION OF MICROPROCESSOR RESOURCES IN A WIRELESS COMMUNICATIONDEVICE (the '369 application) filed Jul. 1, 1999, assigned to theassignee of the present invention and incorporated in its entiretyherein by reference. The values of these parameters can affect theduration of a search.

In one embodiment of the invention, the controller 50 divides PN spaceinto segments. A segment of PN space is a set of PN offsets. Afterdividing the entire PN space into segments each segment is searchedusing coarse search parameters. PN offsets, corresponding to portions ofsegments which have sufficient pilot signal strength, are stored forevaluation and possible further searching. Using coarse searchparameters decreases the time required to perform a search, allowing theentire PN space to be searched quickly, identifying PN offsets of viablepilot signals. Following coarse searching, the PN offsets containingviable pilot signals are searched using fine search parameters. PN spacemay be divided into any desired number of segments, such as for example8 segments. In addition, the segments may be of equal or unequal size.

In one embodiment of the invention, the controller 50 transfers thecoarse search parameters to the search engine 44 for each segmentsequentially. The search engine 44 then performs a search of a PNsegment and analyzes the results. If the signal strength at a PN offsetexceeds a threshold, the measured signal strength value andcorresponding PN offset are stored in memory 49 for later evaluation bythe controller 50. In one embodiment of the invention, the threshold isa set value, resulting in a variable number of PN offsets being storedby the search engine 44 for later evaluation. In another embodiment, thethreshold is adjusted during the search process resulting in a fixednumber of values being stored, such as for example the four strongestsignal levels discovered in a PN segment. After the entire PN space hasbeen searched, the controller 50 evaluates the search results stored inmemory 49. Evaluation of the search results determines which portions ofthe PN space have the highest probability of containing a viable pilotsignal such as, for example, those portions of the PN space with thehighest measured signal level. The controller 50 then adjusts the searchparameters such as, for example, selecting search windows correspondingto the portions of the PN space identified above.

FIG. 5 shows a representation of one embodiment of a strategy fordividing PN space into segments. As described above, each base stationtransmits the same PN sequence as a pilot signal. Each base station in ageographic region transmits the pilot signal at a unique offset. Asnoted above, according to IS-95, the pilot signal is a sequence having2¹⁵ (32.768) PN chips and repeating every 26.66 milliseconds (ms). Thus,the pilot signal can be offset from the remote unit reference by anyvalue from zero to 32,767 chips. Because the PN sequence is cyclic,repeating every 32,768 chips, it can be represented as a circle 58. ThePN reference of the remote unit is located on the circle at therightmost point 59 of the circle 58. Point 59 of circle 58 represents anoffset of zero from the local PN reference in the remote unit. Rotationaround the circle in a counterclockwise direction represents increasingPN offset from the local PN reference 59. In the embodiment of FIG. 5,PN space is divided into eight equal segments. In other embodiments, PNspace may be divided into a different number of segments, and thesegments may be of unequal size. In the embodiment shown in FIG. 5, theboundaries of the segments occur at PN offset values of 4095, 8191,12,287, 16,383, 20,479, 24,575, 28,672 and 32,767 PN chips. When the PNoffset has increased to a value of 32,768 chips, one complete PN periodhas been scanned, and continuing to search with increasing offsets above32,767 PN chips results in a repeated search of the same PN space. Thus,a PN offset of 32,768 chips is equivalent to a PN offset to zero.

Continuing to increase PN offset continues rotation around the circle asdescribed above. In one embodiment, as the search engine scans thesegments of PN space, a set of maximum signal levels and thecorresponding PN offsets are recorded. The number of values stored canbe of any desired size, such as, for example, four values per segment.

FIG. 6 shows an exemplifying set of multiple signal instances of pilotsignals from multiple base stations, relative to PN space segments,arriving at a remote unit. The horizontal axis of FIG. 6 represents PNspace. The vertical axis represents the power received at the remoteunit in decibels (dB). The groups of peaks 60, 62 and 64 representtransmissions from three different base stations in proximity to theremote unit. As shown, the signal from each base station experiences adifferent multipath environment. Also, each base station has a differentPN offset with respect to the remote units local PN reference 66. Thehorizontal axis of FIG. 6 is a linear representation of PN space.Similar to the circular representation of PN space, shown in FIG. 5, thehorizontal axis is divided into eight equal segments. The boundaries ofthe segments correspond to PN offsets of 4095, 8191, 12,287, 16,383,20,479, 24,575, 28,672, and 32,767 PN chips. As in the circularrepresentation, once the PN offset has reached 32,767 chips, one entirePN period 68 has been scanned.

FIG. 7 is an expanded view of the first two segments of PN spacerepresented in FIG. 6. The vertical axis represents the power receivedat the remote unit in decibels (dB). The horizontal axis represents PNspace. Because FIG. 7 shows the first two segments of PN spacerepresented in FIG. 6, the horizontal axis of FIG. 7 corresponds to PNoffsets zero to 8191 and shows pilot signal 60 of one base station. Inone embodiment of the invention, each segment of PN space is scanned bythe search engine and the four strongest signal levels found in eachsegment, and the corresponding PN offsets, are recorded. In thisembodiment, the search engine scans the first segment, PN offset zero to4095, by stepping through various PN offsets.

During the search, the search engine 44 uses a set of coarse searchparameters passed to the search engine 44 by the controller 50. Thecoarse search parameters are selected to achieve a desired searchduration for a given segment of PN space. Search parameters may beselected in accordance with the invention described in theabove-referenced '369 application. For example, the number ofnon-coherent passes may be set to 1, and the integration interval may beset to 0.1 msec. As the search engine 44 steps through the PN segmentusing the coarse search parameters, at each offset, the signal level ofthe pilot signal is measured. In one embodiment, where the threshold isadjusted, the four strongest signal levels of a PN segment are recorded.As shown in FIG. 7, in the first segment, the four strongest signallevels are represented by points 70, 72, 74 and 76. The search engine 44records the signal level and PN offset for each point 70, 72, 74 and 76.This set of values is passed to memory for later evaluation by thecontroller 50.

The search engine 44 then scans the second segment, PN offset from 4096to 8191, by increasing the PN offset, and stepping through the PNsegment. At each offset, the signal level of the pilot signal ismeasured. In the second segment, the four strongest signal levels arerepresented by points 80, 82, 84 and 86. The search engine 44 recordsthe pilot signal level and PN offset for each point 80, 82, 84 and 86.The search engine 44 repeats this process for the remaining segmentsuntil the entire PN space has been scanned.

Decreasing the search duration may result in a decrease in the accuracyof the measurement of the pilot signal by the search engine. Searchparameters selected by the controller 50, for use by the search engine44, are based on a tradeoff between search accuracy and search speed.Therefore, different coarse search parameters may be selected dependingon, for example, the multipath environment the remote unit is operatingin.

FIG. 7 illustrates a condition where an instance of a pilot multipathsignal was not identified during the coarse search. As shown in FIG. 7,during the coarse search of the second segment of PN space, a multipathsignal 88 of group 60 was missed. The signal level of the multipathsignal 88 may have been low during measurement by the search engine dueto, for example, a fade. Due to the signal level of multipath instance88 being low, and the coarse search being completed before the fadeterminates, the measured signal strength of peaks 84 and 86, which arenot pilot signal instances, exceeded peak 88. Thus, the coarse searchdid not identify all the viable multipath instance of pilot signal group60.

As discussed above, in a set of coarse search parameters, the number ofnon-coherent passes and the integration interval are typically decreasedin order to decrease the amount of time required to complete a search.Because the search parameters are selected to decrease the searchduration, the search engine 44 may occasionally miss a multipath signalpeak due to, for example, a local fade, as exemplified in FIG. 7. Thus,for example, if the search engine 44 only takes one non-coherent pass ofthe PN space, and the integration period is set to 0.1 msec, anill-times local fade as shown in FIG. 7 can result in obtaining aninaccurate measure of the average pilot signal level at that offset. Insome situations, for example, if all the multipath instances of a pilotsignal fade at the same time, a viable pilot signal may not be detectedby the coarse search. However, in a typical situation, not all multipathinstances will fade together. Therefore, some multipath instances of allviable pilot signals should be identified for subsequent searching usingfine search parameters.

In one embodiment, following completion of searching all segments,memory 49 has stored a set of 32 peak signal levels, comprising fourpeaks in each of the eight segments, and the corresponding PN offsets.The controller 50 evaluates the set of 32 peak entries to determine themost likely location of pilot signals in the PN space. For example, thecontroller 50 may determine that the PN offset between peaks 80 and 82is small enough, and the signal strength is strong enough, that thesepeaks are two instances of the same pilot signal that have undergonedifferent delays due to the multipath environment. For example, in oneembodiment the controller 50 may only evaluate peaks which exceed −15 dBsignal strength. In another embodiment, the controller 50 may evaluate aset number of peaks, for example, the 32 strongest peaks found duringsearches.

In yet another embodiment, the controller 50 selects a fine search toinclude the entire region of possible reception offsets corresponding toa base station which would produce a peak at an offset found during thecoarse search. For example, if a coarse peak is discovered which has anoffset corresponding to a particular base station, a fine search isconducted over the entire range of PN offsets over which it is likely todetect that particular base station signal.

Evaluating the data from segments one and two in FIG. 7, it can bedetermined that all four points in segment one, 70, 72, 74, 76 andpoints 84 and 86 in segment two are relatively low signal levelscompared to points 80 and 82 in segment two. Thus, the controller 50 maydetermine that there is a higher probability that a pilot signal ispresent in segment two, at the PN offset corresponding to points 80 and82, than at other locations in segments one and two. Although one of themultipath signal peaks 88 of group 60 was missed during the fast search,two of the remaining peaks 80 and 82 were detected. Thus, unless all themultipath signals of a base station are at a low power levelsimultaneously, such as, for example, due to fading, the likelihood isthat at least one of the multipath components of the pilot signal willbe detected.

After determining the portions of PN space with a higher probability ofcontaining a viable pilot signal, the controller 50 selects searchwindows where fine searching will be performed. In one embodiment, thecontroller 50 may select a search window, for performing a fine search,centered about the PN offset of peak 80 wide enough to encompass peak82. In another embodiment, the controller 50 may select a search windowthat begins 3 chips before the PN offset for peak 80 and includesincreasing PN offsets of 6 chips.

Evaluation by the controller 50 is performed on the data from all eightsegments. Following evaluation of the search results, the controller 50is able to select search windows as described above. In addition,because the pilot signal power level has been measured, other searchparameters, such as for example integration interval and number ofnoncoherent passes, may be sent to the search engine, in accordance withthe invention described in the above-referenced '369 application.

In accordance with the '369 application, the controller 50 selectssearch parameters for the search engine to use in a “fine” search. Inone embodiment the fine search may be focused on the portions of PNspace identified during evaluation of the coarse search results, ashaving a high probability of containing a pilot signal. In anotherembodiment the portions of PN space which are searched using the finesearch parameters are identified by a preferred base station andcommunicated to the remote unit.

As described above, the controller 50 may select a search window basedupon the region of the PN space identified during the coarse search ashaving the highest probability of containing a pilot signal. Thecontroller 50 may also select a number of non-coherent passes and anintegration interval based in part on the power level of the pilotsignal measured during the coarse search, and the number of portions ofPN space that are to be searched. For example, the controller 50 mayselect an integration interval in the range of 0.1 msec and 2non-coherent passes if the pilot signal power level was measured at −13dB during the coarse search. In another example, if the pilot signalpower level was measured at −15 dB during the coarse search, thecontroller 50 may select an integration interval of 0.06 msec and 8non-coherent passes. Further details on selecting search parameters maybe found in the above-identified '369 application.

FIG. 8 is a flowchart of an embodiment of the invention illustrating theprocess of performing a coarse search. Flow begins in block 100. Inblock 102, the search parameters for the coarse search are initialized.As discussed in the '369 application, various parameters, such as forexample integration interval and number of non-coherent passes make up aset of search parameters. Flow then continues to block 104 where thecoarse search parameters are sent from the controller 50 to the searchengine 44. In block 106, the search engine 44, using the coarse searchparameters, performs a search of the PN space specified by the searchparameters. Flow continues to block 108 where it is determined if theentire segment has been searched. If the entire segment has not beensearched, flow continues to block 106 where the search engine 44continues to search. If in block 108 it is determined that an entiresegment has been searched, flow continues to block 110. In block 110 thesearch engine 44 passes the number of measured signal strengths and thecorresponding PN offsets of the peak values, as specified by thecontroller 50, to memory 49 for later evaluation by the controller 50.Flow then continues to block 112.

In block 112 it is determined if the entire PN space has been searched.If the entire PN space has not been searched, flow continues to block104 and search parameters for the next PN segment are downloaded to thesearch engine 44. If in block 112 it is determined that the entire PNspace has been searched, flow continues to block 114. In block 114 thecontroller 50 evaluates the search results transferred from the searchengine 44 to memory 49 during search of the PN space. Followingevaluation, flow continues to block 116 where the results from theevaluation of the coarse scan results are used by the controller 50, andin accordance with the above-referenced '369 application, the controller50 then selects appropriate parameters for use by the search engine 44during fine search operations.

More information concerning the searching process, demodulating elementassignment and search engines can be found in:

-   -   (1) U.S. Pat. No. 5,644,591, entitled METHOD AND APPARATUS FOR        PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONS SYSTEM;    -   (2) U.S. Pat. No. 5,805,648, entitled METHOD AND APPARATUS FOR        PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONS SYSTEM;    -   (3) U.S. Pat. Nos. 5,867,527 and 5,710,768, entitled METHOD OF        SEARCHING FOR A BURSTY SIGNAL;    -   (4) U.S. Pat. No. 5,764,687, entitled MOBILE DEMODULATOR        ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION        SYSTEM;    -   (5) U.S. Pat. No. 5,577,022, entitled PILOT SIGNAL SEARCHING        TECHNIQUE FOR A CELLULAR COMMUNICATIONS SYSTEM;    -   (6) U.S. Pat. No. 5,577,022, entitled CELL SITE DEMODULATION        ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION        SYSTEMS;    -   (7) application No. 08/987,172, entitled MULTI CHANNEL        DEMODULATOR, filed on Dec. 9, 1997; and    -   (8) application No. 09/283,010, entitled PROGRAMMABLE MATCHED        FILTER SEARCHER, filed on Mar. 31, 1999;        each of which is assigned to the assignee hereof and        incorporated herein by reference, in its entirety.

Thus, the invention overcomes the longstanding problems in the art ofquickly acquiring an appropriate base station if the wireless link islost by performing a coarse, fast, search of the entire PN space andthen concentrating searches on selected portions of PN space having ahigher probability of containing a viable pilot signal.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears, the invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiment is to be considered in all respects only asillustrative and not restrictive and the scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A wireless communication system comprising: a plurality of basestations, each configured to transmit a PN encoded pilot signal at adifferent time offset than other base stations; and at least one remoteunit configured to perform a coarse search of an entire PN space inaccordance with a set of coarse search parameters, to use results of thecoarse search of the entire PN space to select portions of the PN spacefor a fine search, and to perform the fine search of the selectedportions of the PN space in accordance with a set of fine searchparameters.
 2. The wireless communication system of claim 1 whereinduring the coarse search, the number of non-coherent passes is reducedin comparison with the fine search.
 3. The wireless communication systemof claim 1 wherein during the coarse search, an integration interval isreduced in comparison with the fine search.
 4. A remote unit in awireless communication system comprising: a search engine configured toreceive a set of coarse search parameters, to conduct a coarse search ofan entire PN space in accordance with the set of coarse searchparameters to detect for signals in the wireless communication system,to output search results for the entire PN space, to receive a set offine search parameters and a selection of portions of the PN space, toconduct a fine search of the portions of the PN space in accordance withthe set of fine search parameters, and to output search results for theportions of the PN space; a memory configured to receive and store thesearch results from the search engine and to output the search results;and a controller configured to pass the set of coarse search parametersto the search engine, to receive the search results for the entire PNspace from the memory, to select the portions of the PN space as likelyto contain a pilot signal based on the search results for the entire PNspace, and to pass the set of fine search parameters and the selectionof the portions of the PN space to the search engine.
 5. The remote unitof claim 4, wherein the set of coarse search parameters comprises anumber of non-coherent passes selected from a range of non-coherentpasses, and wherein the number of non-coherent passes in the set ofcoarse search parameters is less than in the set of fine searchparameters.
 6. The remote unit of claim 4, wherein the set of coarsesearch parameters comprises an integration interval selected from aplurality of predetermined integration intervals, and wherein theintegration interval in the set of coarse search parameters is less thanin the set of fine search parameters.
 7. The remote unit of claim 4,wherein the entire PN space is divided into equal segments, and whereinfor the coarse search the search engine is configured to search onesegment at a time until all segments are searched.
 8. The remote unit ofclaim 4, wherein the entire PN space is divided into unequal segments,and wherein for the coarse search the search engine is configured tosearch one segment at a time until all segments are searched.
 9. Theremote unit of claim 4, wherein the portions of the PN space are forpeaks exceeding a predetermined signal strength identified by the searchresults for the entire PN space.
 10. The remote unit of claim 4, whereinthe portions of the PN space are for a predetermined number of strongestpeaks identified by the search results for the entire PN space.
 11. Theremote unit of claim 4, wherein the portions of the PN space are for aregion of possible PN offsets for a base station.
 12. The remote unit ofclaim 4, wherein the portions of the PN space are identified by apreferred base station.
 13. A method of initially acquiring a basestation by a system remote unit, the method comprising: selecting a setof coarse search parameters comprising a number of noncoherent passesand an integration interval; conducting a coarse search of an entire PNspace for a pilot signal according to the set of coarse searchparameters; storing results of the coarse search in a memory; selectingportions of the entire PN space based on the results of the coarsesearch of the entire PN space; and conducting a fine search of theselected portions of the entire PN space according to a set of finesearch parameters.
 14. The method of claim 13 wherein the number ofnon-coherent passes in the coarse search is less than in the finesearch.
 15. The method of claim 13 wherein the integration interval inthe coarse search is less than in the fine search.
 16. The method ofclaim 13 further comprising: storing all measured signal levelsidentified during the coarse search which exceed a threshold level andcorresponding PN offsets.
 17. A remote unit in a wireless communicationsystem configured to perform a search for a pilot signal, the remoteunit comprising: means for selecting a set of coarse search parameterscomprising a number of noncoherent passes and an integration interval;means for conducting a coarse search of an entire PN space for a pilotsignal according to the set of coarse search parameters; means forstoring results of the coarse search; means for selecting portions ofthe entire PN space based on the results of the coarse search of theentire PN space; and means for conducting a fine search of the selectedportions of the entire PN space according to a set of fine searchparameters.
 18. A method of searching in a wireless communication systemthe method comprising: transmitting a PN encoded pilot signal from aplurality of base stations, each base station configured to transmitsaid PN encoded pilot signal at a different time offset than other basestations; performing a coarse search of an entire PN space in accordancewith a set of coarse search parameters by at least one remote unit;selecting portions of the entire PN space based on results of the coarsesearch of the entire PN space; and performing a fine search of theselected portions of the entire PN space in accordance with a set offine search parameters.